This invention re lates generally to Analog to Digital Converters (ADC's) and more particularly to Multi-Rate ADC's.
Analog to Digital Converters (ADC's) are used in a myriad electronic devices. An example of one type of device that incorporates an ADC is a multi-mode receiver. A multi-mode receiver is used in telecommunication equipment such as a mobile telephone or digital tuner.
Multimode receivers use crystal oscillators to generate a single output frequency referred to as a clock frequency. Different carrier signals require different clock frequencies in order to demodulate them. The clock frequencies and other protocols vary according to different standards. For example, the GSM standard requires cocks to operate at 270.833 kHz or at multiples thereof, whereas Wideband Coded Divisional Multiplexed Access standard (WCDMA) operates at 3.84 MHz or multiples thereof. Because there is no direct way of deriving one frequency from another, multi-mode devices, operating at different standards, need to have separate clocks capable of generating different clock reference frequencies. So called multi-clock arrangements are not only expensive, but also add size and complexity to portable devices, such as mobile telephones.
Increasingly, because demands placed upon channel carrying capacity are becoming more exacting the requirements imposed by international Telecommunication Standards on such features as sensitivity level, are also becoming more stringent. Another drawback of such multi clock arrangements is that there is an increased risk of cross-talk and frequency mixing. When this occurs an intolerable amount of noise may be introduced into a system, to such a degree, that the performance of the ADC may not meet the Signal to Noise Ratio (SNR) and noise floor levels and may not allow telephones to meet the sensitivity levels demanded by some of the standards.